Contaminant Data — All Analytes
704 well testing events on record, covering
23 analytes.
Data spans 1955 to 2021.
| Contaminant |
Samples |
Years |
Detection Rate |
Distribution LowModHigh |
vs. Limit |
vs. NY Avg |
|
Radon
|
48 |
2005–2020
|
100% |
|
115% of limit
|
~ typical
|
|
Iron
|
86 |
1955–2020
|
92% |
|
40% of limit
|
↓ 79% below
|
|
Manganese
|
81 |
1955–2020
|
83% |
|
40% of limit
|
↓ 87% below
|
|
Sulfate
|
106 |
1955–2020
|
100% |
|
14% of limit
|
~ typical
|
|
Chloride
|
687 |
1955–2020
|
100% |
|
5% of limit
|
↓ 82% below
|
|
Nitrate
|
96 |
1955–2020
|
75% |
|
4% of limit
|
↓ 70% below
|
|
Uranium
|
49 |
2005–2020
|
98% |
|
2% of limit
|
~ typical
|
|
Arsenic
|
65 |
1970–2020
|
83% |
|
4% of limit
|
↓ 77% below
|
|
Fluoride
|
93 |
1955–2020
|
98% |
|
8% of limit
|
↑ 43% above
|
|
HFPO-DA (GenX)
municipal
|
28 |
2023–2025
|
0% |
|
— |
— |
|
Lead
|
65 |
1970–2020
|
55% |
|
2% of limit
|
↓ 59% below
|
|
Nitrite
|
56 |
1970–2020
|
25% |
|
0% of limit
|
↓ 76% below
|
|
PFHxS
municipal
|
28 |
2023–2025
|
0% |
|
— |
— |
|
PFNA
municipal
|
28 |
2023–2025
|
0% |
|
— |
— |
|
PFOA
municipal
|
28 |
2023–2025
|
0% |
|
— |
— |
|
PFOS
municipal
|
28 |
2023–2025
|
0% |
|
— |
— |
|
E. coli
|
15 |
2005–2020
|
2% |
|
— |
~ typical
|
|
Fecal Coliform
|
14 |
2010–2020
|
3% |
|
— |
~ typical
|
|
Hardness
|
88 |
1955–2020
|
100% |
|
— |
~ typical
|
|
PFBS
municipal
|
28 |
2023–2025
|
0% |
|
— |
— |
|
pH
|
106 |
1955–2020
|
100% |
|
— |
~ typical
|
|
Sodium
|
88 |
1955–2020
|
100% |
|
— |
↓ 67% below
|
|
Total Coliform
|
15 |
2005–2020
|
11% |
|
— |
~ typical
|
Distribution shows the share of samples in each concentration band relative to the EPA Maximum Contaminant Level (MCL): Low = below half the MCL, Moderate = between half and the MCL, High = above the MCL. Analytes without an MCL (e.g. sodium, pH) show — in the limit columns. State average is based on county median values across NY.
Compare your result with St. Lawrence County
Of the 65 arsenic tests on record for St. Lawrence County (1970–2020),
17% found none, and half were below 0.3 µg/L.
Of the 687 chloride tests on record for St. Lawrence County (1955–2020),
0% found none, and half were below 13 mg/L.
Of the 93 fluoride tests on record for St. Lawrence County (1955–2020),
2% found none, and half were below 320 µg/L.
Of the 86 iron tests on record for St. Lawrence County (1955–2020),
8% found none, and half were below 98.2 µg/L.
Of the 65 lead tests on record for St. Lawrence County (1970–2020),
45% found none, and half were below 40 ng/L.
Of the 81 manganese tests on record for St. Lawrence County (1955–2020),
17% found none, and half were below 10.1 µg/L.
Of the 96 nitrate tests on record for St. Lawrence County (1955–2020),
25% found none, and half were below 188 µg/L.
Of the 106 sulfate tests on record for St. Lawrence County (1955–2020),
0% found none, and half were below 36 mg/L.
Of the 49 uranium tests on record for St. Lawrence County (2005–2020),
2% found none, and half were below 0.475 µg/L.
Data Sources
This report aggregates data from the following public databases:
- Water Quality Portal (WQP) — groundwater sample results for St. Lawrence County from EPA, USGS, and state agencies. Browse WQP data →
- EPA Safe Drinking Water Information System (SDWIS) — public water system inventory used to estimate the share of residents on private wells. Browse SDWIS →
- Census ACS 5-Year Estimates — well and cistern household counts (variable DP04_0074E) used to calculate the private-well percentage for St. Lawrence County. Census data explorer →
- USGS Aquifer Atlas — principal aquifer type underlying St. Lawrence County, which informs which contaminants are geologically likely. USGS Aquifer Atlas →
- EPA UCMR 5 (2023–2025) — PFAS occurrence data from public water systems, used as a proxy indicator for aquifer PFAS exposure. Rows marked "municipal" in the table above come from this source. About UCMR 5 →
- EPA Toxics Release Inventory (TRI) — industrial facility chemical releases near St. Lawrence County, used in contamination risk scoring. TRI Explorer →
- EPA Superfund NPL — National Priorities List sites in or near St. Lawrence County, used in contamination risk scoring. Superfund site search →
- CDC PLACES — county-level health outcome prevalence (kidney disease, cancer, heart disease) correlated with detected contaminants. About CDC PLACES →
Methodology
Raw records are downloaded from the Water Quality Portal and normalized to µg/L (ppb). Records are deduplicated by sample ID and date, and certified outliers are excluded. Analyte names are mapped to EPA canonical forms. Detection rates, distribution bands, and MCL comparisons are computed from the normalized dataset.
Distribution bands use the EPA Maximum Contaminant Level as the threshold: concentrations below 50% of the MCL are classed as Low, between 50% and 100% as Moderate, and above 100% as High. For analytes without an MCL (sodium, hardness, pH), distribution is not computed.
State comparison uses the median of county median values across all counties in NY with at least one sample for that analyte.
Last updated: 2026-08-04
Full methodology →